1. Field of the Invention
The present invention relates to long-wavelength VCSELs, methods for manufacturing same, and more particularly to a method for manufacturing such VCSELs with a flip-bonding process.
2. Related Art
Vertical cavity surface emitting lasers (VCSELs) at 1.3 xcexcm-1.55 xcexcm wavelength are ideal single-frequency laser sources for use in high-speed optical communications systems, including wavelength division multiplexed (WDM) applications. The advantages of VCSELs for optical communications are related to their small electrically active volume that allows direct modulation of light output at rates beyond 1 Gbps, and a circular output aperture that enables good coupling to single-mode optical fiber. However, commercially available VCSELs operate in the short wavelength range ( less than 900 nm) making them impractical for fiber lengths over 100 m, or incompatible with a system already operating at the longer wavelengths. The main challenges facing commercialization of VCSELs operating at 1.3 xcexcm or 1.55 xcexcm are the sensitivity of these lasers to thermally induced effects that severely degrade their performance compared to short wavelength GaAs based devices. A particular challenge for VCSELs operating at 1.3 xcexcm is to achieve continuous wave (CW) operation at room temperature. The problem is compounded by the fact that the InGaAsP/InP material system has poor thermal conductivity while the differential gain and quantum efficiency are more sensitive to changes in temperature compared to AlGaAs/GaAs based material.
However, efforts to achieve room-temperature operation at 1.3 xcexcm wavelength have yielded success using two different approaches. These approaches are: (1) Dielectric multilayer mirror stacks on both sides of the active region, and (2) Single or double wafer fused interfaces to bond AlGaAs/GaAs based mirror stacks onto the InP based active region. Both approaches circumvent the need to grow high reflectivity Bragg mirrors using the InP/InGaAsP layers which have a small index difference and poor thermal properties. The wafer fused approach provides high-reflectivity AlGaAs/GaAs mirrors with low diffraction losses and low thermal resistance. Also, current can be injected uniformly into the active region via the doped AlGaAs/GaAs mirror stacks. The wafer-fusion based VCSELs thus represent a significant breakthrough solving mirror problems plaguing InP-based VCSELs. To date, fabrication has been achieved for 1.3 xcexcm wavelength room temperature operating VCSELs using double-fused AlGaAs/GaAs mirrors, (See Qian, et al., xe2x80x9c1.3 xcexcm Vertical-Cavity Surface-Emitting Lasers with Double-Bonded GaAsAlAs Bragg Mirrorsxe2x80x9d IEEE Phot. Tech. Lett., Vol. 9, 8 (1997)) and for single fused oxygen-implanted confinement region with dielectric top mirror (See Qian, et al., xe2x80x9cLong Wavelength (1.3 xcexcm) Vertical-Cavity Surface-Emitting Lasers with a Wafer-Bonded Mirror and an Oxygen-Implanted Confinement Regionxe2x80x9d Appl. Phys. Lett. Vol. 71, 25 (1997)).
The other successful approach for room temperature CW operation at 1.3 xcexcm wavelength uses a dielectric multi-layer stack on both sides of an InGaAsP multiple quantum well (MQW) based active region (See Uchiyama, et al., xe2x80x9cContinuous-Wave Operation up to 36 xc2x0 C. of 1.3 xcexcm GaInAsP-InP Vertical-Cavity Surface-Emitting Lasersxe2x80x9d IEEE Phot. Tech. Lett. Vol. 9, 141 (1997)). In this design high reflectivity mirrors are dielectric stacks (See Spaziani, et al., xe2x80x9cHigh-Performance Substrate-Removed InGaAs Schottky Photodetectorsxe2x80x9d IEEE Phot. Tech. Lett. Vol. 10, 1144 (1998)), typically made of Si/Al2O3, Si/SiO2 or ZnSe/MgF2 layers. Current is injected under the dielectric through a ring metal contact to an InGaAs contact layer. The maximum temperature of CW operation was 35xc2x0 C.
The traditional double dielectric stack based VCSEL is schematically shown in FIG. 1. The VCSEL 10 is formed on a diaphragm approximately 2 xcexcm thick in the direction shown by arrows A. The device includes an MQW active region 12, in-grown n-DBR 13, a mesa formed on p-DBR 14, a p-type ring contact 16 formed thereon and a top dielectric stack 18 extending therethrough. The diaphragm sits on an InP substrate 20, which includes an n-type contact 22 on the bottom thereof. A bottom dielectric stack 24 extends from the underside of the wafer.
VCSELs fabricated according to the prior-art model exhibit a very high series resistance as the current has to travel almost 100 xcexcm through a thin diaphragm to reach the active region. This thin diaphragm leads to poor mechanical stability and low thermal conductivity making room temperature CW operation difficult to achieve.
What is needed, and has not heretofore been developed, is a low cost, high yield method for producing high performance InP based VCSELs which can achieve continuous wave operation at room temperature.
It is a primary object of the invention to provide a method for producing high performance InP based VCSELs operating at wavelengths used in optical communications systems and networks.
It is an additional object to provide high performance VCSELs which can achieve continuous wave operation at temperatures higher than room temperature.
It is a further object of the invention to provide a low cost method of producing high performance VCSELs.
It is a still further object of the invention to provide high performance VCSELs with reduced series resistance.
It is yet another object of the invention to provide high performance VCSELs having improved mechanical rigidity.
It is still another object of the invention to provide a method for making long-wavelength VCSELs using a flip-bonding technique.
The present invention provides a method of manufacturing VCSELs that involves a flip-bonding process wherein the top surface of the VCSEL wafer is bonded face down onto a surrogate substrate. The process begins in a manner similar to traditional double dielectric stack based VCSEL, but then involves flip-bonding the wafer onto a metal (for example, indium or silver), or a metal-loaded epoxy coated surrogate substrate. The InP substrate is then removed by selective etching. After flip-bonding, the wafer fabrication proceeds on the freshly etched surface which now forms the top surface. Next, standard mesa-isolation and contact formation techniques are performed on this newly etched surface.